Display device and electronic device

ABSTRACT

A transistor whose channel region includes an oxide semiconductor is used as a pull down transistor. The band gap of the oxide semiconductor is 2.0 eV or more, preferably 2.5 eV or more, more preferably 3.0 eV or more. Thus, hot carrier degradation in the transistor can be suppressed. Accordingly, the circuit size of the semiconductor device including the pull down transistor can be made small. Further, a gate of a pull up transistor is made to be in a floating state by switching of on/off of the transistor whose channel region includes an oxide semiconductor. Note that when the oxide semiconductor is highly purified, the off-state current of the transistor can be 1 aA/μm (1×10 −18  A/μm) or less. Therefore, the drive capability of the semiconductor device can be improved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/026,863, filed Feb. 14, 2011, now allowed, which claims the benefitof a foreign priority application filed in Japan as Serial No.2010-033669 on Feb. 18, 2010, both of which are incorporated byreference.

TECHNICAL FIELD

One embodiment of the present invention relates to display devices. Forexample, one embodiment of the present invention relates to liquidcrystal display devices. One of the technical fields relates to displaydevices in which images are displayed when pixels are selected by gatesignal lines and source signal lines. Further, one of the technicalfields relates to semiconductor devices such as driver circuits used indisplay devices and electronic devices including display devices.

BACKGROUND ART

Gate driver circuits including amorphous silicon transistors (alsoreferred to as a-Si TFTs) have been developed. Such a gate drivercircuit has a problem of malfunctions due to a shift in the thresholdvoltage of a transistor for keeping the potential of a gate line low (atan L level) (such a transistor is also referred to as a pull downtransistor). In order to solve this problem, a gate driver circuit inwhich a pull down transistor is repeatedly turned on and off in a periodduring which the potential of a gate line is kept low has been disclosed(see References 1 and 2, for example). With such a gate driver circuit,a period during which the pull down transistor is on can be shortened;thus, deterioration of the pull down transistor can be suppressed.

In addition, the gate driver circuit including amorphous silicontransistors includes a transistor for controlling timing of outputtinghigh voltage to the gate line (such a transistor is also referred to asa pull up transistor). One of a source and a drain of the pull uptransistor is connected to a clock signal line. The other of the sourceand the drain of the pull up transistor is connected to a gate signalline. A driving method by which the potential of a gate of the pull uptransistor is made higher than the high (H-level) potential of a clocksignal by capacitive coupling is employed. In order to realize thedriving method, it is necessary to make the gate of the pull uptransistor be in a floating state. Thus, it is necessary to turn off allthe transistors that are connected to the gate of the pull uptransistor.

REFERENCE

-   [Reference 1] Japanese Published Patent Application No. 2007-207413-   [Reference 2] Japanese Published Patent Application No. 2008-009393

DISCLOSURE OF INVENTION

In a conventional technique, in order that a pull down transistor may berepeatedly turned on and off, a circuit for controlling the on-off ofthe pull down transistor is needed. Thus, there is a limit to thedecrease in the circuit size of a semiconductor device. In addition,even when all the transistors that are connected to a gate of the pullup transistor are turned off, electrical charges accumulated in the gateof the pull up transistor are lost due to the off-state current of thetransistor as time passes. Therefore, it is difficult to lower the drivefrequency of a semiconductor device such as a gate driver circuit.Further, the range of drive frequency at which the semiconductor devicecan operate is narrowed. Accordingly, there is a limit to improvement inthe drive capability of the semiconductor device.

In view of the foregoing problems, an object of one embodiment of thepresent invention is to make the circuit size of a semiconductor devicesmall. Further, an object of one embodiment of the present invention isto improve the drive capability of the semiconductor device. Note thatin one embodiment of the present invention, there is no need to achieveall the objects.

The objects can be achieved when a transistor whose channel regionincludes an oxide semiconductor is used as the pull up transistor or thepull down transistor. Note that the oxide semiconductor is an oxidesemiconductor which is highly purified by drastic removal of impurities(hydrogen, water, and the like) which serve as electron donors.

Note that the band gap of the oxide semiconductor is 2.0 eV or more,preferably 2.5 eV or more, more preferably 3.0 eV or more. Thus, in thetransistor whose channel region includes the oxide semiconductor, impactionization and avalanche breakdown do not easily occur. That is,carriers (electrons) in the oxide semiconductor are not easilyaccelerated. Therefore, in the transistor whose channel region includesthe oxide semiconductor, fluctuations in the threshold voltage of thetransistor due to injection of carriers (electrons) into a gateinsulating layer (so-called hot carrier degradation) can be suppressed.

Further, the number of carriers in the transistor whose channel regionincludes the oxide semiconductor is extremely small. Thus, off-statecurrent per micrometer of the channel width can be 1 aA (1×10⁻¹⁸ A) orless. This off-state current is represented as 1 aA/μm.

In other words, one embodiment of the present invention is a displaydevice which includes a plurality of gate signal lines, a plurality ofsource signal lines, a pixel provided in a region where the gate signalline and the source signal line intersect with each other, and a gatedriver circuit electrically connected to the plurality of gate signallines. The gate driver circuit includes a first transistor, a secondtransistor, and an inverter circuit. A first terminal of the firsttransistor is electrically connected to a first wiring, and a secondterminal of the first transistor is electrically connected to a secondwiring. A first terminal of the second transistor is electricallyconnected to a third wiring, and a second terminal of the secondtransistor is electrically connected to the second wiring. An inputterminal of the inverter circuit is electrically connected to a gate ofthe first transistor, and an output terminal of the inverter circuit iselectrically connected to a gate of the second transistor. A channelregion of each of the first transistor and the second transistorincludes an oxide semiconductor. The off-state current of each of thefirst transistor and the second transistor is 1 aA/μm or less.

One embodiment of the present invention is a display device whichincludes a plurality of gate signal lines, a plurality of source signallines, a pixel provided in a region where the gate signal line and thesource signal line intersect with each other, and a gate driver circuitelectrically connected to the plurality of gate signal lines. The gatedriver circuit includes a first transistor, a second transistor, and aninverter circuit. A first terminal of the first transistor iselectrically connected to a first wiring, and a second terminal of thefirst transistor is electrically connected to a second wiring. A firstterminal of the second transistor is electrically connected to a thirdwiring, and a second terminal of the second transistor is electricallyconnected to a gate of the first transistor. An input terminal of theinverter circuit is electrically connected to the gate of the firsttransistor, and an output terminal of the inverter circuit iselectrically connected to a gate of the second transistor. A channelregion of each of the first transistor and the second transistorincludes an oxide semiconductor. The off-state current of each of thefirst transistor and the second transistor is 1 aA/μm or less.

One embodiment of the present invention is a display device whichincludes a plurality of gate signal lines, a plurality of source signallines, a pixel provided in a region where the gate signal line and thesource signal line intersect with each other, and a gate driver circuitelectrically connected to the plurality of gate signal lines. The gatedriver circuit includes a first transistor, a second transistor, a thirdtransistor, and an inverter circuit. A first terminal of the firsttransistor is electrically connected to a first wiring, and a secondterminal of the first transistor is electrically connected to a secondwiring. A first terminal of the second transistor is electricallyconnected to a third wiring, and a second terminal of the secondtransistor is electrically connected to the second wiring. A firstterminal of the third transistor is electrically connected to a fourthwiring; a second terminal of the third transistor is electricallyconnected to a gate of the first transistor; and a gate of the thirdtransistor is electrically connected to the fourth wiring. An inputterminal of the inverter circuit is electrically connected to the gateof the first transistor, and an output terminal of the inverter circuitis electrically connected to a gate of the second transistor. A channelregion of each of the first to third transistors includes an oxidesemiconductor. The off-state current of each of the first to thirdtransistors is 1 aA/μm or less.

One embodiment of the present invention is a display device whichincludes a plurality of gate signal lines, a plurality of source signallines, a pixel provided in a region where the gate signal line and thesource signal line intersect with each other, and a gate driver circuitelectrically connected to the plurality of gate signal lines. The gatedriver circuit includes a first transistor, a second transistor, a thirdtransistor, and an inverter circuit. A first terminal of the firsttransistor is electrically connected to a first wiring, and a secondterminal of the first transistor is electrically connected to a secondwiring. A first terminal of the second transistor is electricallyconnected to a third wiring, and a second terminal of the secondtransistor is electrically connected to the second wiring. A firstterminal of the third transistor is electrically connected to the thirdwiring; a second terminal of the third transistor is electricallyconnected to a gate of the first transistor; and a gate of the thirdtransistor is electrically connected to a fourth wiring. An inputterminal of the inverter circuit is electrically connected to the gateof the first transistor, and an output terminal of the inverter circuitis electrically connected to a gate of the second transistor. A channelregion of each of the first to third transistors includes an oxidesemiconductor. The off-state current of each of the first to thirdtransistors is 1 aA/μm or less.

One embodiment of the present invention is a display device whichincludes a plurality of gate signal lines, a plurality of source signallines, a pixel provided in a region where the gate signal line and thesource signal line intersect with each other, and a gate driver circuitelectrically connected to the plurality of gate signal lines. The gatedriver circuit includes a first transistor, a second transistor, a thirdtransistor, a fourth transistor, and an inverter circuit. A firstterminal of the first transistor is electrically connected to a firstwiring, and a second terminal of the first transistor is electricallyconnected to a second wiring. A first terminal of the second transistoris electrically connected to a third wiring, and a second terminal ofthe second transistor is electrically connected to the second wiring. Afirst terminal of the third transistor is electrically connected to afourth wiring; a second terminal of the third transistor is electricallyconnected to a gate of the first transistor; and a gate of the thirdtransistor is electrically connected to the fourth wiring. A firstterminal of the fourth transistor is electrically connected to the thirdwiring; a second terminal of the fourth transistor is electricallyconnected to the gate of the first transistor; and a gate of the fourthtransistor is electrically connected to a fifth wiring. An inputterminal of the inverter circuit is electrically connected to the gateof the first transistor, and an output terminal of the inverter circuitis electrically connected to a gate of the second transistor. A channelregion of each of the first to fourth transistors includes an oxidesemiconductor. The off-state current of each of the first to fourthtransistors is 1 aA/μm or less.

One embodiment of the present invention is an electronic deviceincluding the display device and an operation switch which controls animage of the display device.

In this specification and the like, when an object is explicitlydescribed in a singular form, the object is preferably singular.However, the present invention is not limited to this, and the objectcan be plural. Similarly, when an object is explicitly described in aplural form, the object is preferably plural. However, the presentinvention is not limited to this, and the object can be singular.

In this specification and the like, terms such as “first”, “second”, and“third” are used for distinguishing various elements, members, regions,layers, and areas from others. Therefore, the terms such as “first”,“second”, and “third” do not limit the number of the elements, members,regions, layers, areas, or the like. Further, for example, the term“first” can be replaced with the term “second”, “third”, or the like.

In one embodiment of the present invention, a transistor whose channelregion includes an oxide semiconductor is used as a pull downtransistor. Thus, hot carrier degradation in the pull down transistorcan be suppressed. Therefore, the number of transistors serving as pulldown transistors can be reduced. Accordingly, the size of a circuit forcontrolling the on-off of a pull down transistor can be made small.Consequently, the circuit size of a semiconductor device including thepull down transistor can be made small.

Further, in one embodiment of the present invention, a gate of a pull uptransistor is made to be in a floating state by switching of the on-offof a transistor whose channel region includes an oxide semiconductor.Thus, electrical charges accumulated in the gate of the pull uptransistor can be held for a long period of time. Therefore, the drivefrequency of a semiconductor device including the pull up transistor canbe lowered. Further, the range of drive frequency at which thesemiconductor device can operate can be broadened. Accordingly, thedrive capability of the semiconductor device can be improved.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A and 1B illustrate structures of circuits in Embodiment 1;

FIG. 2A is a timing chart for illustrating operation of the circuit inEmbodiment 1, and

FIG. 2B is a schematic view for illustrating operation of the circuit inEmbodiment 1;

FIGS. 3A and 3B are schematic views for illustrating operation of thecircuit in Embodiment 1;

FIGS. 4A and 4B are schematic views for illustrating operation of thecircuit in Embodiment 1;

FIGS. 5A to 5C illustrate structures of circuits in Embodiment 1;

FIGS. 6A to 6C illustrate structures of circuits in Embodiment 1;

FIGS. 7A and 7B illustrate structures of circuits in Embodiment 1;

FIGS. 8A to 8C illustrate structures of circuits in Embodiment 1;

FIGS. 9A and 9B are timing charts for illustrating operation of thecircuit in Embodiment 1;

FIGS. 10A to 10D illustrate structures of circuits in Embodiment 1;

FIG. 11 illustrates a structure of a shift register circuit inEmbodiment 2;

FIG. 12 is a timing chart for illustrating operation of the shiftregister circuit in Embodiment 2;

FIGS. 13A to 13D are examples of diagrams for illustrating steps ofmanufacturing a transistor in Embodiment 3;

FIGS. 14A to 14C illustrate structures of display devices in Embodiment4;

FIGS. 15A to 15H illustrate devices for realizing the technical idea ofthe present invention; and

FIGS. 16A to 16H illustrate devices for realizing the technical idea ofthe present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described below with reference to the drawings. Notethat the embodiments can be implemented in various different ways. Itwill be readily appreciated by those skilled in the art that modes anddetails of the embodiments can be changed in various ways withoutdeparting from the spirit and scope of the present invention. Therefore,the present invention should not be construed as being limited to thedescription of the embodiments. Note that in structures described below,the same portions or portions having similar functions are denoted bycommon reference numerals in different drawings, and detaileddescription thereof is not repeated. In the reference drawings, thesize, the thickness of layers, or regions are exaggerated for clarity insome cases. Therefore, the embodiments of the present invention are notlimited to such scales.

Embodiment 1

In this embodiment, a circuit in a display device which is oneembodiment of the present invention is described.

FIG. 1A illustrates a structure example of a circuit which includes atransistor 101, a transistor 102, a transistor 103, a transistor 104, atransistor 105, and a circuit 200. The transistors included in thecircuit illustrated in FIG. 1A are n-channel transistors. An n-channeltransistor is turned on when a potential difference between a gate and asource is higher than the threshold voltage.

Note that an oxide semiconductor may be used for a semiconductor layerof the transistor included in the circuit illustrated in FIG. 1A. It ispreferable that the oxide semiconductor be an intrinsic (i-type) orsubstantially intrinsic oxide semiconductor which is obtained bysufficiently lowering a hydrogen concentration to be highly purified andhas sufficiently low carrier density. With the oxide semiconductor, thesubthreshold swing of the transistor can be decreased. The off-statecurrent of the transistor can be reduced. The withstand voltage of thetransistor can be improved. The temperature characteristics of thetransistor can be improved. Deterioration of the transistor can besuppressed. Specifically, the amount of shifts in the threshold voltageof the transistor can be reduced.

Note that the oxide semiconductor can be used for semiconductor layersof some transistors, and a semiconductor which is different from theoxide semiconductor (for example, silicon (e.g., amorphous silicon,microcrystalline silicon, or polycrystalline silicon), an organicsemiconductor, or the like) can be used for semiconductor layers of theother transistors. Note that the oxide semiconductor is used for atleast a semiconductor layer of a transistor whose source or drain isconnected to a gate of the transistor 101.

The connection relations in the circuit illustrated in FIG. 1A aredescribed. A first terminal of the transistor 101 is connected to awiring 111. A second terminal of the transistor 101 is connected to awiring 112. A first terminal of the transistor 102 is connected to awiring 113. A second terminal of the transistor 102 is connected to thewiring 112. A first terminal of the transistor 103 is connected to thewiring 113. A second terminal of the transistor 103 is connected to agate of the transistor 101. A gate of the transistor 103 is connected toa gate of the transistor 102. A first terminal of the transistor 104 isconnected to a wiring 114. A second terminal of the transistor 104 isconnected to the gate of the transistor 101. A gate of the transistor104 is connected to the wiring 114. A first terminal of the transistor105 is connected to the wiring 113. A second terminal of the transistor105 is connected to the gate of the transistor 101. A gate of thetransistor 105 is connected to a wiring 115. An input terminal of thecircuit 200 is connected to the gate of the transistor 101. An outputterminal of the circuit 200 is connected to the gate of the transistor102. Note that the gate of the transistor 101 is denoted by a node 11,and the gate of the transistor 102 is denoted by a node 12. Note thatthe circuit 200 can be connected to a given wiring or a given nodedepending on its structure. For example, the circuit 200 can beconnected to one or more of the wiring 111, the wiring 112, the wiring113, the wiring 114, the wiring 115, the node 11, and the node 12.

Note that since a source and a drain of a transistor change depending onthe structure, the operating condition, and the like of the transistor,it is difficult to define which is a source or a drain. Therefore, inthis document, one of a source and a drain is referred to as a firstterminal and the other thereof is referred to as a second terminal.

An example of the structure of the circuit 200 is described withreference to FIG. 1B. The circuit 200 includes a transistor 201, atransistor 202, a transistor 203, and a transistor 204. A first terminalof the transistor 201 is connected to a wiring 116. A second terminal ofthe transistor 201 is connected to the node 12. A first terminal of thetransistor 202 is connected to the wiring 113. A second terminal of thetransistor 202 is connected to the node 12. A gate of the transistor 202is connected to the node 11. A first terminal of the transistor 203 isconnected to the wiring 116. A second terminal of the transistor 203 isconnected to a gate of the transistor 201. A gate of the transistor 203is connected to the wiring 116. A first terminal of the transistor 204is connected to the wiring 113. A second terminal of the transistor 204is connected to the gate of the transistor 201. A gate of the transistor204 is connected to the node 11.

A clock signal is input to the wiring 111. An output signal of thecircuit in this embodiment is input to the wiring 112. Voltage V₂ issupplied to the wiring 113. A start pulse is input to the wiring 114. Areset signal is input to the wiring 115. Voltage V₁ is supplied to thewiring 116. Here, the H-level potential of the signal input to thewiring 111, the wiring 112, the wiring 114, and the wiring 115 isreferred to as a potential V₁ for convenience, and the L-level potentialof the signal input to the wiring 111, the wiring 112, the wiring 114,and the wiring 115 is referred to as a potential V₂ for convenience.

The wiring 111 is used for transmitting a signal such as a clock signalfrom an external circuit such as a controller to the circuit in thisembodiment. The wiring 111 functions as a signal line or a clock signalline. The wiring 112 is used for transmitting an output signal of thecircuit in this embodiment to a circuit such as a pixel circuit or ademultiplexer. The wiring 112 functions as a signal line or a gatesignal line. The wiring 113 is used for supplying power supply voltagesuch as the voltage V₂ from an external circuit such as a power supplycircuit to the circuit in this embodiment. The wiring 113 functions as apower supply line, a negative power supply line, or a ground line. Thewiring 114 is used for transmitting a start signal from another circuitor an external circuit such as a timing controller to the circuit inthis embodiment. The wiring 114 functions as a signal line. The wiring115 is used for transmitting a reset signal from another circuit or anexternal circuit such as a timing controller to the circuit in thisembodiment. The wiring 115 functions as a signal line. The wiring 116 isused for supplying power supply voltage such as the voltage V₁ from anexternal circuit such as a power supply circuit to the circuit in thisembodiment. The wiring 116 functions as a power supply line or apositive power supply line.

The transistor 101 functions as a switch for controlling electricalcontinuity between the wiring 111 and the wiring 112. Further, thetransistor 101 has a function of controlling timing of raising thepotential of the node 11 by capacitive coupling between the secondterminal and the gate of the transistor 101. The transistor 102functions as a switch for controlling electrical continuity between thewiring 113 and the wiring 112. The transistor 103 functions as a switchfor controlling electrical continuity between the wiring 113 and thenode 11. The transistor 104 functions as a switch for controllingelectrical continuity between the wiring 114 and the node 11. Further,the transistor 104 functions as a diode whose input terminal isconnected to the wiring 114 and whose output terminal is connected tothe node 11. The transistor 105 functions as a switch for controllingelectrical continuity between the wiring 113 and the node 11. Thetransistor 201 functions as a switch for controlling electricalcontinuity between the wiring 116 and the node 12. Further, thetransistor 201 has a function of controlling timing of raising thepotential of a node 21 by capacitive coupling between the secondterminal and the gate of the transistor 201. The transistor 202functions as a switch for controlling electrical continuity between thewiring 113 and the node 12. The transistor 203 functions as a switch forcontrolling electrical continuity between the wiring 116 and the node12. Further, the transistor 203 functions as a diode whose inputterminal is connected to the wiring 116 and whose output terminal isconnected to the node 21. The transistor 204 functions as a switch forcontrolling electrical continuity between the wiring 113 and the node21.

The circuit 200 functions as a control circuit for controlling thepotential of the node 12 and the on-off of the transistor 102 and thetransistor 103. Further, the circuit 200 functions as an invertercircuit for inverting the potential of the node 11 and outputting theinverted potential to the node 12.

Next, an example of the operation of the circuits in FIGS. 1A and 1B isdescribed with reference to a timing chart in FIG. 2A. Here, forexample, the circuit in FIG. 1B is described. The timing chart in FIG.2A includes a period A, a period B, a period C, and a period D.

In the period A, the potential of the wiring 111 (potential V₁₁₁) is atV₂; the potential of the wiring 114 (potential V₁₁₄) is at V₁; and thepotential of the wiring 115 (potential V₁₁₅) is at V₂. Thus, thetransistor 104 is turned on, so that electrical continuity between thewiring 114 and the node 11 is established. The transistor 105 is turnedoff. At this time, the circuit 200 sets the potential of the node 12(potential V₁₂) at V₂. Thus, the transistor 102 is turned off, so thatelectrical continuity between the wiring 113 and the wiring 112 is notestablished. The transistor 103 is turned off, so that electricalcontinuity between the wiring 113 and the node 11 is not established.Thus, the potential of the wiring 114 is supplied to the node 11, sothat the potential of the node 11 (potential V₁₁) starts to rise. Then,the potential of the node 11 exceeds V₂+V_(th101) (V_(th101) representsthe threshold voltage of the transistor 101). Thus, the transistor 101is turned on, so that electrical continuity between the wiring 111 andthe wiring 112 is established. Accordingly, the potential of the wiring111 is supplied to the wiring 112, so that the potential of the wiring112 (potential V₁₁₂) is at V₂ (see FIG. 2B).

After that, the potential of the node 11 continuously rises. Then, thepotential of the node 11 reaches V₁−V_(th104) (V_(th104) represents thethreshold voltage of the transistor 104). Thus, the transistor 104 isturned off, so that electrical continuity between the wiring 114 and thenode 11 is not established. Accordingly, the node 11 is made to be in afloating state, so that the potential of the node 11 is kept atV₁−V_(th104) (V₁−V_(th104) is higher than V₂+V_(th101)) (see FIG. 3A).

In the period B, the potential of the wiring 111 is at V₁; the potentialof the wiring 114 is at V₂; and the potential of the wiring 115 is keptat V₂. Thus, the transistor 104 is kept off, so that electricalcontinuity between the wiring 114 and the node 11 remains unestablished.The transistor 105 is kept off. At this time, the circuit 200continuously sets the potential of the node 12 at V₂. Thus, thetransistor 102 is kept off, so that electrical continuity between thewiring 113 and the wiring 112 remains unestablished. The transistor 103is kept off, so that electrical continuity between the wiring 113 andthe node 11 remains unestablished. Thus, the node 11 is kept in afloating state, so that the potential of the node 11 is kept atV₁−V_(th104). Thus, the transistor 101 is kept on, so that electricalcontinuity between the wiring 111 and the wiring 112 remainsestablished. At this time, the potential of the wiring 111 is at V₁.Thus, the potential of the wiring 112 starts to rise. Then, since thenode 11 is in a floating state, the potential of the node 11 starts torise by parasitic capacitance between the gate and the second terminalof the transistor 101. Finally, the potential of the node 11 reachesV₁+V_(th101)+V_(a) (V_(a) is a positive potential). Accordingly, thepotential of the wiring 112 can rise to V₁ (see FIG. 3B). Such operationis referred to as bootstrap operation.

In the period C, the potential of the wiring 111 is at V₂; the potentialof the wiring 114 is kept at V₂; and the potential of the wiring 115 isat V₁. Thus, the transistor 104 is kept off, so that electricalcontinuity between the wiring 114 and the node 11 remains unestablished.The transistor 105 is turned on, so that electrical continuity betweenthe wiring 113 and the node 11 is established. Thus, the potential ofthe wiring 113 is supplied to the node 11, so that the potential of thenode 11 is at V₂. Thus, the transistor 101 is turned off, so thatelectrical continuity between the wiring 111 and the wiring 112 is notestablished. At this time, the circuit 200 sets the potential of thenode 12 at V₁. Thus, the transistor 102 is turned on, so that electricalcontinuity between the wiring 113 and the wiring 112 is established. Thetransistor 103 is turned on, so that electrical continuity between thewiring 113 and the node 11 is established. Thus, the potential of thewiring 113 is supplied to the wiring 112, so that the potential of thewiring 112 is at V₂ (see FIG. 4A).

In the period D, the potential of the wiring 111 is repeatedly at V₁ andV₂ in turn; the potential of the wiring 114 is kept at V₂; and thepotential of the wiring 115 is at V₂. Thus, the transistor 104 is keptoff, so that electrical continuity between the wiring 114 and the node11 remains unestablished. The transistor 105 is turned off, so thatelectrical continuity between the wiring 113 and the node 11 is notestablished. At this time, the circuit 200 continuously sets thepotential of the node 12 at V₁. Thus, the transistor 102 is kept on, sothat electrical continuity between the wiring 113 and the wiring 112remains established. The transistor 103 is kept on, so that electricalcontinuity between the wiring 113 and the node 11 remains established.Thus, the potential of the wiring 113 is continuously supplied to thenode 11, so that the potential of the node 11 is kept at V₂. Thus, thetransistor 101 is kept off, so that electrical continuity between thewiring 111 and the wiring 112 remains unestablished. Thus, the potentialof the wiring 113 is continuously supplied to the wiring 112, so thatthe potential of the wiring 112 is kept at V₂ (see FIG. 4B).

Next, the operation of the circuit 200 is specifically described. Forexample, the potential of the node 11 is higher than or equal toV₂+V_(th202) (V_(th202) represents the threshold voltage of thetransistor 202) and higher than or equal to V₂+V_(th204) (V_(th204)represents the threshold voltage of the transistor 204). Thus, thetransistor 202 is turned on, so that electrical continuity between thewiring 113 and the node 12 is established. The transistor 204 is turnedon, so that electrical continuity between the wiring 113 and the node 21is established. At this time, the transistor 203 is turned on, so thatelectrical continuity between the wiring 116 and the node 21 isestablished. Thus, the potential of the wiring 116 and the potential ofthe wiring 113 are supplied to the node 21, so that the potential of thenode 21 (potential V₂₁) is higher than V₂ and lower than V₁. Thepotential of the node 21 is determined by the current supply capability(e.g., channel length, channel width, and mobility) of the transistor203 and the current supply capability of the transistor 204. Here, thepotential of the node 21 is lower than V₂+V_(th201) (V_(th201)represents the threshold voltage of the transistor 201). Thus, thetransistor 201 is turned off, so that electrical continuity between thewiring 116 and the node 12 is not established. Thus, the potential ofthe wiring 113 is supplied to the node 12, so that the potential of thenode 12 is at V₂ (for example, in the period A and the period B).

In contrast, for example, the potential of the node 11 is lower thanV₂+V_(th202) and lower than V₂+V_(th204). Thus, the transistor 202 isturned off, so that electrical continuity between the wiring 113 and thenode 12 is not established. The transistor 204 is turned off, so thatelectrical continuity between the wiring 113 and the node 21 is notestablished. At this time, the transistor 203 is turned on, so thatelectrical continuity between the wiring 116 and the node 21 isestablished. Thus, the potential of the wiring 116 is supplied to thenode 21, so that the potential of the node 21 rises. Finally, thepotential of the node 21 is at V₁+V_(th201)+V_(b) (V_(b) is a positivepotential). Thus, the transistor 201 is turned on, so that electricalcontinuity between the wiring 116 and the node 12 is established. Thus,the potential of the wiring 116 is supplied to the node 12, so that thepotential of the node 12 is at V₁ (for example, in the period C and theperiod D).

As described above, in the circuits illustrated in FIGS. 1A and 1B, thepotential of the wiring 112 can be made equal to the potential of thewiring 111 by the bootstrap operation. Further, in the period B, apotential difference between the gate and the source of the transistor101 (V_(gs)) can be increased, so that the rise time of V112 can beshortened.

Note that in a conventional semiconductor device, the subthreshold swingof a transistor is large. Thus, it takes a longer time from when thepotential of the wiring 114 is at V₁ until when the transistor 104 isturned on. Further, the problems of the conventional semiconductordevice are as follows. The length of the period A needs to be madelonger; thus, it is difficult to raise drive frequency. The rise time ofV112 is long (the rise time of an output signal is long). A load whichcan be connected to the wiring 112 is decreased. The channel width ofthe transistor 101 is increased. The layout area is increased.

In contrast, in this embodiment, the subthreshold swing of a transistoris small. Thus, drive capability can be improved. For example, when thesubthreshold swing of the transistor 104 is small, it is possible toshorten the time from when the potential of the wiring 114 is at V₁until when the transistor 104 is turned on. Thus, the length of theperiod A can be shortened. Accordingly, drive frequency can be improved.As another example, when the subthreshold swing of the transistor 104 issmall, it is possible to shorten the rise time of the potential of thewiring 112. In addition, even when a large load is connected to thewiring 112, the load can be driven. Further, the channel width of thetransistor 101 can be decreased; thus, the layout area can be decreased.

Note that in the conventional semiconductor device, the off-statecurrent of the transistor is high. Thus, the amount of electricalcharges that are lost from the node 11 as time passes is large. Further,the problems of the conventional semiconductor device are as follows.The potential of the node 11 is decreased. The time during which thepotential of the node 11 can be kept higher than a potential at whichthe transistor 101 is turned on is short. It is difficult to lower drivefrequency. The range of drive frequency at which the semiconductordevice can operate is narrowed.

In contrast, in this embodiment, the off-state current of the transistoris low. Thus, drive capability can be improved. For example, when theoff-state current of the transistor 103, the transistor 104, and thetransistor 105 is low, the amount of electrical charges that are lostfrom the node 11 can be decreased. Thus, the decrease in the potentialof the node 11 can be suppressed. That is, the time during which thepotential of the node 11 can be kept higher than the potential at whichthe transistor 101 is turned on can be extended. Accordingly, the drivefrequency can be lowered; thus, the range of drive frequency at whichthe semiconductor device can operate can be broadened.

Note that in the conventional semiconductor device, the transistoreasily deteriorates and the amount of shifts in the threshold voltage ofthe transistor is large. Thus, the transistor is driven so as to berepeatedly turned on and off. Further, the problems of the conventionalsemiconductor device are as follows. Two transistors are connected inparallel and are alternately turned on. A circuit for controlling theon-off of the transistors is complicated. The number of transistors isincreased. In order to suppress deterioration of the transistors, it isnecessary to make the channel widths of the transistors large. Further,in order to suppress deterioration of the transistors, it is necessaryto make the channel lengths of the transistors long. The layout area isincreased.

In contrast, in this embodiment, the amount of shifts in the thresholdvoltage of the transistor is small. Thus, drive capability can beimproved. For example, when the amount of shifts in the thresholdvoltage of the transistor 102 and the transistor 103 is small, the timeduring which these transistors are on can be extended. Therefore, acircuit for controlling the on-off of the transistor 102 and thetransistor 103 can be simplified. Accordingly, the number of transistorscan be decreased; thus, the layout area can be decreased. Further, whenthe amount of shifts in the threshold voltage of the transistor 102 andthe transistor 103 is small, the channel widths or channel lengths ofthese transistors can be decreased. Thus, the layout area can bedecreased. Further, when the amount of shifts in the threshold voltageof the transistors is small, the time during which the semiconductordevice can operate can be extended.

The circuit in the display device which is one embodiment of the presentinvention is not limited to the circuits in FIGS. 1A and 1B. Circuitswith a variety of structures can be used. Examples of circuits aredescribed below.

For example, in the circuits illustrated in FIGS. 1A and 1B, the inputterminal of the circuit 200 can be connected to the wiring 112, asillustrated in FIG. 5A. Specifically, the gate of the transistor 202 andthe gate of the transistor 204 can be connected to the wiring 112. Notethat FIG. 5A illustrates the case where the input terminal of thecircuit 200 is connected to the wiring 112 in the circuit illustrated inFIG. 1A.

As another example, in the circuits illustrated in FIGS. 1A and 1B andFIG. 5A, the first terminal of the transistor 103 can be connected tothe wiring 112 and the gate of the transistor 103 can be connected tothe wiring 111, as illustrated in FIG. 5B. Thus, the time during whichthe transistor 103 is on can be shortened, so that deterioration of thetransistor 103 can be suppressed. Further, in the period B, thepotential of the node 11 can be prevented from being too high.Accordingly, a transistor electrically connected to the node 11 (e.g.,the transistor 101, the transistor 104, the transistor 105, or thetransistor included in the circuit 200) can be prevented from beingdamaged or deteriorating, for example. Note that FIG. 5B illustrates thecase where the first terminal of the transistor 103 is connected to thewiring 112 and the gate of the transistor 103 is connected to the wiring111 in the circuit illustrated in FIG. 1A.

As another example, in the circuits illustrated in FIGS. 1A and 1B andFIGS. 5A and 5B, the first terminal of the transistor 104 can beconnected to the wiring 116, as illustrated in FIG. 5C. Note that FIG.5C illustrates the case where the first terminal of the transistor 104is connected to the wiring 116 in the circuit illustrated in FIG. 1A.

A variety of elements such as transistors and capacitors can be providedin the circuits illustrated in FIGS. 1A and 1B and FIGS. 5A to 5C.Examples of circuits are described below.

For example, in the circuits illustrated in FIGS. 1A and 1B and FIGS. 5Ato 5C, a transistor 121 can be provided, as illustrated in FIG. 6A. Afirst terminal of the transistor 121 is connected to the wiring 113; asecond terminal of the transistor 121 is connected to the wiring 112;and a gate of the transistor 121 is connected to the wiring 115. In theperiod C, the transistor 121 is turned on, so that the potential of thewiring 113 is supplied to the wiring 112. Thus, the fall time of V112can be shortened. Note that FIG. 6A illustrates the case where thetransistor 121 is provided in the circuit illustrated in FIG. 1A.

As another example, in the circuits illustrated in FIGS. 1A and 1B,FIGS. 5A to 5C, and FIG. 6A, a transistor 122 can be provided, asillustrated in FIG. 6B. A first terminal of the transistor 122 isconnected to the wiring 113; a second terminal of the transistor 122 isconnected to the node 12; and a gate of the transistor 122 is connectedto the wiring 114. In the period A, the transistor 122 is turned on, sothat the potential of the wiring 113 is supplied to the node 12. Thus,the fall time of V12 can be shortened, so that timing of turning off thetransistor 103 can be earlier. Accordingly, timing of when the potentialof the node 11 reaches V₁−V_(th104) can be earlier; thus, the length ofthe period A can be shortened. Therefore, drive frequency can be raised.Note that FIG. 6B illustrates the case where the transistor 122 isprovided in the circuit illustrated in FIG. 1A.

As another example, in the circuits illustrated in FIGS. 1A and 1B,FIGS. 5A to 5C, and FIGS. 6A and 6B, a transistor 123 can be provided,as illustrated in FIG. 6C. A first terminal of the transistor 123 isconnected to the wiring 116; a second terminal of the transistor 123 isconnected to the node 12; and a gate of the transistor 123 is connectedto the wiring 115. In the period C, the transistor 123 is turned on, sothat the potential of the wiring 116 is supplied to the node 12. Thus,in the period C, the fall time of V12 can be shortened. Therefore,timing of turning on the transistor 102 and the transistor 103 can beearlier. Accordingly, timing of supplying the potential of the wiring113 to the wiring 112 can be earlier; thus, the fall time of thepotential of the wiring 112 can be shortened. Note that FIG. 6Cillustrates the case where the transistor 123 is provided in the circuitillustrated in FIG. 1A.

As another example, in the circuits illustrated in FIGS. 1A and 1B,FIGS. 5A to 5C, and FIGS. 6A to 6C, a transistor 124 and a transistor125 can be provided, as illustrated in FIG. 7A. A first terminal of thetransistor 124 is connected to the wiring 111; a second terminal of thetransistor 124 is connected to a wiring 117; and a gate of thetransistor 124 is connected to the node 11. A first terminal of thetransistor 125 is connected to the wiring 113; a second terminal of thetransistor 125 is connected to the wiring 117; and a gate of thetransistor 125 is connected to the node 12. Thus, the potential of thewiring 117 and the potential of the wiring 112 can be changed at thesame timing. For example, it is preferable that one of the wiring 112and the wiring 117 be connected to a load and the other of the wiring112 and the wiring 117 be connected to a different circuit. Note that itis possible not to provide the transistor 125. Note that FIG. 7Aillustrates the case where the transistor 124 and the transistor 125 areprovided in the circuit illustrated in FIG. 1A.

As another example, in the circuits illustrated in FIGS. 1A and 1B,FIGS. 5A to 5C, FIGS. 6A to 6C, and FIG. 7A, a capacitor 126 can beprovided between the gate and the second terminal of the transistor 101,as illustrated in FIG. 7B. Note that the capacitor 126 can be providedbetween the gate and the second terminal of the transistor 124. Notethat FIG. 7B illustrates the case where the capacitor 126 is provided inthe circuit illustrated in FIG. 1A.

The structure of the circuit 200 is not limited to the structureillustrated in FIG. 1B. A variety of different structures can beemployed. Examples of different structures are described. For example,as illustrated in FIG. 8A, it is possible not to provide the transistor201 and the transistor 202. Note that in the circuit 200 illustrated inFIG. 8A, the gate of the transistor 203 can be connected to the node 12,as illustrated in FIG. 8B. Further, in the circuit 200 illustrated inFIG. 8A, the gate of the transistor 203 can be connected to a wiring118, as illustrated in FIG. 8C. A signal obtained by inversion of asignal input to the wiring 111 (such a signal is referred to as aninversion clock signal) or a signal that is out of phase with the signalinput to the wiring 111 (e.g., a signal that is out of phase with thesignal input to the wiring 111 by 180°, 90°, or 45°) is input to thewiring 118. Thus, the wiring 118 functions as a signal line, a clocksignal line, or an inversion clock signal line. Note that the structureof the circuit 200 is not limited to the above structure as long as thefunctions of the circuit 200 can be realized.

The timing chart of the circuit is not limited to the timing chartillustrated in FIG. 2A. A variety of timing charts can be used. Examplesof timing charts are described. For example, a signal input to thewiring 111 can be non-balanced, as illustrated in FIG. 9A. Thus, in theperiod C, timing of when the potential of the wiring 115 becomes V₁ canbe later than timing of when the potential of the wiring 111 becomes V₂.Accordingly, the fall time of V112 can be shortened. As another example,a signal input to the wiring 111 can be a multiphase clock signal, asillustrated in FIG. 9B. Thus, power consumption can be reduced. Notethat FIG. 9B is an example of a timing chart when a four-phase clocksignal is input to the wiring 111.

The W/L (W: channel width and L: channel length) ratio of the transistor101 is preferably higher than the W/L ratios of the transistor 102, thetransistor 103, the transistor 104, and the transistor 105.Specifically, the W/L ratio of the transistor 101 is preferably 1.5 to10 times the W/L ratio of the transistor 104. More preferably, the W/Lratio of the transistor 101 is 1.8 to 7 times the W/L ratio of thetransistor 104. Still more preferably, the W/L ratio of the transistor101 is 2 to 4 times the W/L ratio of the transistor 104. Further, theW/L ratio of the transistor 102 is preferably higher than the W/L ratioof the transistor 103 because a load of the transistor 103 (e.g., thenode 11) is smaller than a load of the transistor 102 (e.g., the wiring112). Specifically, the W/L ratio of the transistor 102 is preferably1.5 to 8 times the W/L ratio of the transistor 103. More preferably, theW/L ratio of the transistor 102 is 2 to 6 times the W/L ratio of thetransistor 103. Still more preferably, the W/L ratio of the transistor102 is 2 to 5 times the W/L ratio of the transistor 103. Furthermore, atleast one of the channel length of the transistor 102 and the channellength of the transistor 103 is preferably longer than the channellength of the transistor 105. Specifically, at least one of the channellength of the transistor 102 and the channel length of the transistor103 is preferably 1 to 4 times the channel length of the transistor 105.More preferably, at least one of the channel length of the transistor102 and the channel length of the transistor 103 is 1.3 to 3 times thechannel length of the transistor 105. Still more preferably, at leastone of the channel length of the transistor 102 and the channel lengthof the transistor 103 is 1.8 to 2.5 times the channel length of thetransistor 105.

The width of the wiring 111 is preferably smaller than at least one ofthe channel width of the transistor 101, the channel width of thetransistor 102, and the channel width of the transistor 104. Further,the width of the wiring 111 is preferably larger than at least one ofthe widths of the wiring 116.

Of the circuits described in this embodiment, each of the followingstructures is included as one embodiment of the present invention: asemiconductor device including the transistor 101, the transistor 102,and the circuit 200 (see FIG. 10A); a semiconductor device including thetransistor 101, the transistor 103, and the circuit 200 (see FIG. 10B);a semiconductor device including the transistor 101, the transistor 102,the transistor 103, and the circuit 200 (see FIG. 10C); and asemiconductor device including the transistor 101, the transistor 102,the transistor 104, and the circuit 200 (see FIG. 10D).

Embodiment 2

In this embodiment, a shift register circuit in a display device whichis one embodiment of the present invention is described. A shiftregister circuit in this embodiment can include any of the circuitsdescribed in Embodiment 1. Further, the shift register circuit in thisembodiment can be used as a driver circuit of a display device, such asa gate driver circuit and/or a source driver circuit.

FIG. 11 illustrates a structure example of a shift register circuitwhich includes N (N is a natural number) pieces of circuits 301(circuits 301_1 to 301_N). Any of the circuits described in Embodiment 1can be used as the circuit 301. FIG. 11 illustrates an example in whichthe circuit illustrated in FIG. 1A is used as the circuit 301.

Connection relations in the shift register circuit illustrated in FIG.11 are described. The connection relation in a circuit 301_i (i is anatural number that is 2 or more and less than N−1) is described as anexample. The circuit 301_i is connected to a wiring 311_i, a wiring311_i−1, a wiring 311_i+1, one of a wiring 312 and a wiring 313, and awiring 314. Specifically, in the circuit 301_i, the wiring 112 isconnected to the wiring 311_i; the wiring 114 is connected to the wiring311_i−1; the wiring 115 is connected to the wiring 311_i+1; the wiring111 is connected to one of the wiring 312 and the wiring 313; and thewiring 113 is connected to the wiring 314. Note that in the case wherethe wiring 111 is connected to the wiring 312 in the circuit 301_i, thewiring 111 is connected to the wiring 313 in a circuit 301_i+1 and acircuit 301_i−1. The circuit 301_1 differs from the circuit 301_i inthat the wiring 114 is connected to a wiring 315. The circuit 301_Ndiffers from the circuit 301_i in that the wiring 115 is connected to anoutput terminal of a dummy circuit (a circuit 301_D). Note that astructure which is similar to the structure of the circuit 301 or partof the structure of the circuit 301 can be used as the structure of thecircuit 301_D.

The operation of the shift register circuit illustrated in FIG. 11 isdescribed with reference to a timing chart in FIG. 12.

The operation of the circuit 301_i is described as an example. First,the potential of the wiring 311_i−1 (potential V₃₁₁ _(_) _(i−1)) is atV₁. Then, the circuit 301_i performs the operation in the period Adescribed in Embodiment 1, so that the potential of the wiring 311_i(potential V₃₁₁ _(_) _(i)) is at V₂. After that, the potential of thewiring 312 (potential V₃₁₂) and the potential of the wiring 313(potential V₃₁₃) are inverted. Then, the circuit 301_i performs theoperation in the period B described in Embodiment 1, so that thepotential of the wiring 311_i is at V₁. After that, the potential of thewiring 312 and the potential of the wiring 313 are inverted, so that thepotential of the wiring 311_i+1 (potential V₃₁₁ _(_) _(i+1)) is at V₁.Then, the circuit 301_i performs the operation in the period C describedin Embodiment 1, so that the potential of the wiring 311_i is at V₂.After that, the circuit 301_i performs the operation in the period Ddescribed in Embodiment 1 until the potential of the wiring 311_i−1 isat V₁ again, so that the potential of the wiring 311_i is kept at V₂.Note that that the circuit 301_1 differs from the circuit 301_i in thatit performs the operation in the period A when the potential of thewiring 315 (potential V₃₁₅) is at V₁. Further, the circuit 301_N differsfrom the circuit 301_i in that it performs the operation in the period Cwhen an output signal of the circuit 301_D is at V₁.

As described above, the potentials of the wirings 311_1 to 311_N(potentials V₃₁₁ _(_) ₁ to V₃₁₁ _(_) _(N)) can be sequentially at V₁.When the circuit described in Embodiment is used in the shift registercircuit illustrated in FIG. 11, the shift register circuit can haveadvantages which are similar to those of the circuit described inEmbodiment 1.

An output signal of the shift register circuit is input to a wiring 311(one of the wirings 311_1 to 311_N. A clock signal is input to thewiring 312. A clock signal that is out of phase with the clock signalinput to the wiring 312 or a signal obtained by inversion of the clocksignal input to the wiring 312 is input to the wiring 313. The voltageV₂ is supplied to the wiring 314. A start signal is input to the wiring315.

The wiring 311 is used for transmitting an output signal of the shiftregister circuit to a circuit such as a pixel circuit or ademultiplexer. The wiring 311 functions as a signal line or a gatesignal line. Each of the wiring 312 and the wiring 313 is used fortransmitting a signal such as a clock signal from an external circuitsuch as a controller to the shift register circuit in this embodiment.Each of the wiring 312 and the wiring 313 functions as a signal line ora clock signal line. The wiring 314 is used for supplying power supplyvoltage such as the voltage V₂ from an external circuit such as a powersupply circuit to the shift register circuit in this embodiment. Thewiring 314 functions as a power supply line, a negative power supplyline, or a ground line. The wiring 315 is used for transmitting a startsignal from an external circuit such as a controller to the shiftregister circuit in this embodiment. The wiring 315 functions as asignal line.

Embodiment 3

In this embodiment, an example of a transistor included in the circuitdescribed in Embodiment 1 or 2 is described. Specifically, examples ofthe structure of a transistor whose channel region includes an oxidesemiconductor and manufacturing steps thereof are described.

As the oxide semiconductor, the following oxides can be used: anIn—Sn—Ga—Zn—O-based oxide semiconductor that is an oxide of four metalelements; an In—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-basedoxide semiconductor, an In—Al—Zn—O-based oxide semiconductor, aSn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxidesemiconductor, or a Sn—Al—Zn—O-based oxide semiconductor that is anoxide of three metal elements; an In—Zn—O-based oxide semiconductor, aSn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor,a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxidesemiconductor, or an In—Mg—O-based oxide semiconductor that is an oxideof two metal elements; an In—O-based oxide semiconductor; a Sn—O-basedoxide semiconductor; a Zn—O-based oxide semiconductor; and the like.Further, SiO₂ may be contained in the oxide semiconductor.

For the oxide semiconductor, a substance represented by InMO₃(ZnO)_(m)(m>0, where m is not a natural number) can be used. Here, M denotes oneor more metal elements selected from Ga, Al, Mn, or Co. For example, Mcan be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like. Among oxidesemiconductor semiconductors whose composition formulae are expressed byInMO₃(ZnO)_(m) (m>0, where m is not a natural number), an oxidesemiconductor which includes Ga as M is referred to as anIn—Ga—Zn—O-based oxide semiconductor, and a thin film of theIn—Ga—Zn—O-based oxide semiconductor is also referred to as anIn—Ga—Zn—O-based film. In addition, an oxide semiconductor materialexpressed by In—Ga—Zn—O in this specification is InGaO₃(ZnO)_(m) (m>0,where m is not a natural number), and it can be confirmed by analysisusing ICP-MS or RBS that m is not a natural number.

An example of a method for manufacturing a transistor whose channelregion includes an oxide semiconductor is described with reference toFIGS. 13A to 13D.

FIGS. 13A to 13D illustrate an example of the cross-sectional structureof a transistor. A transistor 410 illustrated in FIG. 13D has a kind ofbottom-gate structure called a channel-etched structure.

Although a single-gate transistor is illustrated in FIG. 13D, amulti-gate transistor including a plurality of channel regions can beformed when needed.

Steps of forming the transistor 410 over a substrate 400 are describedbelow with reference to FIGS. 13A to 13D.

First, a conductive film is formed over the substrate 400 having aninsulating surface. Then, a gate electrode layer 411 is formed through afirst photolithography process.

Although there is no particular limitation on a substrate which can beused as the substrate 400 having an insulating surface, it is necessarythat the substrate have at least heat resistance high enough towithstand heat treatment to be performed later. For example, a glasssubstrate including barium borosilicate glass, aluminoborosilicateglass, or the like can be used. In the case where the temperature of theheat treatment to be performed later is high, a glass substrate whosestrain point is 730° C. or higher is preferably used.

An insulating film serving as a base film may be provided between thesubstrate 400 and the gate electrode layer 411. The base film has afunction of preventing diffusion of an impurity element from thesubstrate 400, and can be formed to have a single-layer structure or alayered structure including one or more films selected from a siliconnitride film, a silicon oxide film, a silicon nitride oxide film, or asilicon oxynitride film.

The gate electrode layer 411 can be formed to have a single-layerstructure or a layered structure including a metal material such asmolybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper,neodymium, or scandium; or an alloy material which contains the metalmaterial as its main component.

Then, a gate insulating layer 402 is formed over the gate electrodelayer 411.

The gate insulating layer 402 can be formed to have a single-layerstructure or a layered structure including a silicon oxide layer, asilicon nitride layer, a silicon oxynitride layer, a silicon nitrideoxide layer, or an aluminum oxide layer by plasma-enhanced CVD,sputtering, or the like. Alternatively, a high-k material such ashafnium oxide (HfO_(x)) or tantalum oxide (TaO_(x)) can be used for thegate insulating layer. The thickness of the gate insulating layer 402 is100 to 500 nm. In the case where the gate insulating layer 402 is formedto have a layered structure, a first gate insulating layer having athickness of 50 to 200 nm and a second gate insulating layer having athickness of 5 to 300 nm are stacked.

In this embodiment, as the gate insulating layer 402, a siliconoxynitride layer is formed to a thickness of 100 nm or less byplasma-enhanced CVD.

Further, as the gate insulating layer 402, a silicon oxynitride layermay be formed using a high-density plasma apparatus. Here, ahigh-density plasma apparatus refers to an apparatus which can realize aplasma density of 1×10¹¹/cm³ or higher. For example, plasma is generatedby application of a microwave power of 3 to 6 kW so that an insulatinglayer is formed. Since the insulating layer formed using thehigh-density plasma apparatus can have a uniform thickness, theinsulating layer has excellent step coverage. Further, as for theinsulating layer formed using the high-density plasma apparatus, thethickness of a thin film can be controlled precisely.

The film quality of the insulating layer formed using the high-densityplasma apparatus is greatly different from that of an insulating layerformed using a conventional parallel plate PCVD apparatus. The etchingrate of the insulating layer formed using the high-density plasmaapparatus is lower than that of the insulating layer formed using theconventional parallel plate PCVD apparatus by 10% or more or 20% or morein the case where the etching rates with the same etchant are comparedto each other. Thus, it can be said that the insulating layer formedusing the high-density plasma apparatus is a dense layer.

An oxide semiconductor (a highly purified oxide semiconductor) which ismade to be intrinsic (i-type) or substantially intrinsic in a later stepis highly sensitive to an interface state and interface charge; thus, aninterface between the oxide semiconductor and the gate insulating layeris important. Thus, the gate insulating layer (GI) which is in contactwith the highly purified oxide semiconductor needs high quality.Therefore, high-density plasma-enhanced CVD using microwaves (2.45 GHz)is preferable because a dense high-quality insulating layer having highwithstand voltage can be formed. This is because when the highlypurified oxide semiconductor is closely in contact with the high-qualitygate insulating layer, the interface state can be reduced and interfaceproperties can be favorable. It is important that the gate insulatinglayer have lower interface state density with an oxide semiconductor anda favorable interface as well as having favorable film quality as a gateinsulating layer.

Then, an oxide semiconductor film 430 is formed to a thickness of 2 to200 nm over the gate insulating layer 402. As the oxide semiconductorfilm 430, an In—Ga—Zn—O-based oxide semiconductor film, an In—Zn—O-basedoxide semiconductor film, or the like is used. In this embodiment, theoxide semiconductor film 430 is deposited by sputtering with the use ofan In—Ga—Zn—O-based oxide semiconductor target. A cross-sectional viewat this stage corresponds to FIG. 13A. Alternatively, the oxidesemiconductor film 430 can be deposited by sputtering in a rare gas(typically argon) atmosphere, an oxygen atmosphere, or an atmosphereincluding a rare gas (typically argon) and oxygen.

Here, deposition is performed using a metal oxide target containing In,Ga, and Zn (In₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio]). The depositioncondition is set as follows: the distance between the substrate and thetarget is 100 mm; the pressure is 0.2 Pa; the direct current (DC) poweris 0.5 kW; and the atmosphere is an atmosphere containing argon andoxygen (argon:oxygen=30 sccm:20 sccm and the flow rate ratio of oxygenis 40%). Note that it is preferable that pulsed direct-current (DC)power be used because powdery substances generated in deposition can bereduced and the film thickness can be uniform. The thickness of anIn—Ga—Zn—O-based film is 5 to 200 nm. In this embodiment, as the oxidesemiconductor film, a 20-nm-thick In—Ga—Zn—O-based film is deposited bysputtering with the use of an In—Ga—Zn—O-based metal oxide target. Next,the oxide semiconductor film 430 is processed into an island-shapedoxide semiconductor layer through a second photolithography process.

Then, the oxide semiconductor layer is dehydrated or dehydrogenated. Thetemperature of first heat treatment for dehydration or dehydrogenationis higher than or equal to 400° C. and lower than or equal to 750° C.,preferably higher than or equal to 400° C. and lower than the strainpoint of the substrate. Here, after the substrate is put in an electricfurnace which is a kind of heat treatment apparatus and heat treatmentis performed on the oxide semiconductor layer at 450° C. for one hour ina nitrogen atmosphere, water and hydrogen are prevented from being mixedinto the oxide semiconductor layer by preventing the substrate frombeing exposed to the air; thus, oxide semiconductor layer 431 isobtained (see FIG. 13B).

Note that the heat treatment apparatus is not limited to an electricfurnace, and may be provided with a device for heating an object to beprocessed by thermal conduction or thermal radiation from a heater suchas a resistance heater. For example, an RTA (rapid thermal annealing)apparatus such as a GRTA (gas rapid thermal annealing) apparatus or anLRTA (lamp rapid thermal annealing) apparatus can be used. An LRTAapparatus is an apparatus for heating an object to be processed byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus with which heat treatment is performedusing a high-temperature gas. As the gas, an inert gas which does notreact with an object to be processed by heat treatment, such as nitrogenor a rare gas such as argon, is used.

For example, as the first heat treatment, GRTA may be performed asfollows. The substrate is transferred and put in an inert gas heated ata high temperature of 650 to 700° C., is heated for several minutes, andis transferred and taken out of the inert gas heated at the hightemperature. GRTA enables high-temperature heat treatment in a shorttime.

Note that in the atmosphere of the first heat treatment, it ispreferable that water, hydrogen, or the like be not contained innitrogen, a rare gas such as helium, neon, or argon, or dry air. Forexample, the purity of nitrogen or a rare gas such as helium, neon, orargon which is introduced into the heat treatment apparatus ispreferably 6N (99.9999%) or higher, more preferably 7N (99.99999%) orhigher (that is, the impurity concentration is 1 ppm or lower,preferably 0.1 ppm or lower).

In addition, the first heat treatment for the oxide semiconductor layercan be performed on the oxide semiconductor film 430 before beingprocessed into the island-shaped oxide semiconductor layer. In thatcase, the substrate is taken out of the heat apparatus after the firstheat treatment, and then the second photolithography process isperformed.

Further, in the case where an opening portion is formed in the gateinsulating layer 402, the formation of the opening portion may beperformed before or after the oxide semiconductor film 430 is dehydratedor dehydrogenated.

Note that the etching of the oxide semiconductor film 430 here is notlimited to wet etching, and dry etching may be employed.

As an etching gas used for dry etching of the oxide semiconductor film430, a gas containing chlorine (e.g., chlorine (Cl₂) or borontrichloride (BCl₃)) is preferably used.

As an etchant used for wet etching of the oxide semiconductor film 430,a solution obtained by mixture of phosphoric acid, acetic acid, andnitric acid, an ammonia hydrogen peroxide mixture (a hydrogen peroxidesolution at 31 wt %: ammonia water at 28 wt %: water=5:2:2), or the likecan be used. Alternatively, ITO-07N (produced by KANTO CHEMICAL CO.,INC.) may be used.

Next, a metal conductive film is formed over the gate insulating layer402 and the oxide semiconductor layer 431. The metal conductive film maybe formed by sputtering or vacuum evaporation. As the material of themetal conductive film, an element selected from aluminum (Al), chromium(Cr), copper (Cu), tantalum (Ta), titanium (Ti), molybdenum (Mo),tungsten (W), neodymium (Nd), or scandium (Sc); an alloy including anyof the elements; an alloy including any of these elements incombination; or the like can be used. Alternatively, a nitride film ofany of the elements may be used. Alternatively, one or more materialsselected from manganese (Mn), magnesium (Mg), zirconium (Zr), beryllium(Be), and yttrium (Y) may be used. Further, the metal conductive filmmay have a single-layer structure or a layered structure of two or morelayers. For example, a single-layer structure of an aluminum filmcontaining silicon, a two-layer structure in which a titanium film isstacked over an aluminum film, a three-layer structure in which atitanium film, an aluminum film, and a titanium film are stacked in thatorder, and the like can be given.

When heat treatment is performed after the formation of the metalconductive film, it is preferable that the metal conductive film haveheat resistance high enough to withstand the heat treatment.

A resist mask is formed over the metal conductive film through a thirdphotolithography process; a source electrode layer 415 a and a drainelectrode layer 415 b are formed by selective etching; then, the resistmask is removed (see FIG. 13C).

In this embodiment, a titanium film is used as the metal conductivefilm, an In—Ga—Zn—O-based oxide is used for the oxide semiconductorlayer 431, and an ammonia hydrogen peroxide solution (a mixture ofammonia, water, and a hydrogen peroxide solution) is used as an etchant.

Note that in the third photolithography process, only part of the oxidesemiconductor layer 431 is etched so that an oxide semiconductor layerhaving a groove (a depression) is formed in some cases.

In order to reduce the number of photomasks used in the photolithographyprocesses and to reduce the number of processes, an etching process maybe performed using a multi-tone mask which is an exposure mask throughwhich light is transmitted to have a plurality of intensities. A resistmask formed using a multi-tone mask has a plurality of thicknesses andcan be changed in shape by ashing; therefore, the resist mask can beused in a plurality of etching processes for processing films intodifferent patterns. Therefore, a resist mask corresponding to at leasttwo or more kinds of different patterns can be formed by one multi-tonemask. Thus, the number of exposure masks and the number of correspondingphotolithography processes can be reduced, so that the process can besimplified.

Next, plasma treatment is performed using a gas such as nitrous oxide(N₂O), nitrogen (N₂), or argon (Ar). With this plasma treatment,absorbed water and the like which attach to a surface of the oxidesemiconductor layer exposed are removed. Alternatively, plasma treatmentmay be performed using a mixture gas of oxygen and argon.

After the plasma treatment, an oxide insulating layer 416 which servesas a protective insulating film and is in contact with part of the oxidesemiconductor layer 431 is formed without exposure to the air.

The oxide insulating layer 416 can be formed to have a thickness of atleast 1 nm or more by a method by which an impurity such as water orhydrogen is not mixed into the oxide insulating layer 416, such assputtering, as appropriate. When hydrogen is contained in the oxideinsulating layer 416, hydrogen enters the oxide semiconductor layer, soa backchannel of the oxide semiconductor layer 431 has lower resistance(has n-type conductivity) and a parasitic channel is formed. Therefore,it is important that a deposition method in which hydrogen is not usedbe employed in order that the oxide insulating layer 416 contain aslittle hydrogen as possible.

In this embodiment, a 200-nm-thick silicon oxide film is deposited asthe oxide insulating layer 416 by sputtering. The substrate temperatureat the time of deposition is in the range of from room temperature to300° C., and 100° C. in this embodiment. The silicon oxide film can bedeposited by sputtering in a rare gas (typically argon) atmosphere, anoxygen atmosphere, or an atmosphere including a rare gas (typicallyargon) and oxygen. Further, a silicon oxide target or a silicon targetcan be used as a target. For example, a silicon oxide film can bedeposited using a silicon target in an atmosphere including oxygen andnitrogen by sputtering.

Next, second heat treatment (preferably at 200 to 400° C., for example,250 to 350° C.) is performed in an inert gas atmosphere, a dry airatmosphere, or an oxygen gas atmosphere. For example, the second heattreatment is performed at 250° C. for one hour in a nitrogen atmosphere.Through the second heat treatment, part of the oxide semiconductor layer(a channel region) is heated while being in contact with the oxideinsulating layer 416. Thus, oxygen is supplied to the part of the oxidesemiconductor layer (the channel region).

Through the above steps, after the heat treatment for dehydration ordehydrogenation is performed on the oxide semiconductor layer, the partof the oxide semiconductor layer (the channel region) is selectivelymade to be in an oxygen excess state. Through the steps, the transistor410 is formed.

Further, heat treatment may be performed at 100 to 200° C. for 1 to 30hours in an air atmosphere. In this embodiment, the heat treatment isperformed at 150° C. for 10 hours. This heat treatment may be performedat a fixed heating temperature. Alternatively, the following change inthe heating temperature may be conducted plural times repeatedly: theheating temperature is increased from room temperature to a temperatureof 100 to 200° C. and then decreased to room temperature.

A protective insulating layer may be formed over the oxide insulatinglayer 416. For example, a silicon nitride film is formed by RFsputtering. Since RF sputtering has high productivity, it is preferablyused as a deposition method of the protective insulating layer. Theprotective insulating layer is formed using an inorganic insulating filmwhich does not contain an impurity such as moisture, a hydrogen ion, andOH⁻ and blocks entry of such an impurity from the outside, typically asilicon nitride film, an aluminum nitride film, a silicon nitride oxidefilm, or an aluminum oxynitride film. In this embodiment, as theprotective insulating layer, a protective insulating layer 403 is formedusing a silicon nitride film (see FIG. 13D).

In this embodiment, the oxide semiconductor layer of the transistor 410is an intrinsic (i-type) or substantially intrinsic oxide semiconductorlayer obtained by removal of hydrogen, which is an n-type impurity, fromthe oxide semiconductor and the increase in purity so that an impurityother than the main components of the oxide semiconductor is notincluded as much as possible. In other words, the oxide semiconductorlayer of the transistor 410 is a highly purified intrinsic (i-type)semiconductor layer or a semiconductor layer which is close to a highlypurified i-type semiconductor layer not by addition of an impurity butby removal of an impurity such as hydrogen or water as much as possible.In this manner, the Fermi level (E_(f)) can be equal to the intrinsicFermi level (E_(i)).

It is said that the band gap (E_(g)) of the oxide semiconductor is 3.15eV and electron affinity (χ) is 4.3 eV. The work function of titanium(Ti) used for the source electrode layer and the drain electrode layeris substantially equal to the electron affinity (χ) of the oxidesemiconductor. In this case, the Schottky electron barrier is not formedat an interface between the metal and the oxide semiconductor.

For example, even in the case of a transistor whose channel width W is1×10⁴ μm and whose channel length L is 3 μm, off-state current at roomtemperature can be 10⁻¹³ A or less and a subthreshold swing can be 0.1V/decade (the thickness of the gate insulating layer is 100 nm).

By the increase in purity so that an impurity other than the maincomponents of the oxide semiconductor is not included as much aspossible in this manner, the transistor 410 can operate favorably.

In order to prevent variation in electrical characteristics of the oxidesemiconductor, an impurity that causes the variation, such as hydrogen,moisture, a hydroxyl group, or hydride (also referred to as a hydrogencompound), is intentionally removed. Additionally, the oxidesemiconductor becomes a highly purified electrically i-type (intrinsic)oxide semiconductor by supply of oxygen which is a main component of theoxide semiconductor that is simultaneously reduced in a step of removingthe impurity.

Therefore, it is preferable that the amount of hydrogen in the oxidesemiconductor be as small as possible. Further, the number of carriersin the highly purified oxide semiconductor is significantly small (closeto zero), and the carrier density is lower than 1×10¹²/cm³, preferably1×10¹¹/cm³ or lower. That is, the carrier density of the oxidesemiconductor layer can be extremely close to zero. Since the number ofcarriers in the oxide semiconductor layer is significantly small, theoff-state current of the transistor can be reduced. It is preferablethat the off-state current be as low as possible. The amount of currentper micrometer of the channel width (W) in the transistor is 100 aA orless, preferably 10 zA (zepto-ampere)/μm or less, more preferably 1zA/μm or less. Further, the transistor has no pn junction and does notdeteriorate due to hot carriers; thus, the electrical characteristics ofthe transistor are not adversely affected.

In a transistor whose channel region includes an oxide semiconductorwhich is highly purified by drastic removal of hydrogen contained in anoxide semiconductor layer as described above, the amount of off-statecurrent can be significantly reduced. In other words, in circuit design,the oxide semiconductor layer can be regarded as an insulator when thetransistor is off. In contrast, it can be estimated that the oxidesemiconductor layer has better current supply capability than asemiconductor layer including amorphous silicon when the transistor ison.

A transistor including low-temperature polysilicon is designed on theassumption that off-state current is about 10000 times that of atransistor including an oxide semiconductor. Therefore, in the casewhere the transistor including an oxide semiconductor is compared withthe transistor including low-temperature polysilicon, the voltage holdtime of the transistor including an oxide semiconductor can be extendedabout 10000 times when storage capacitances are equal or substantiallyequal to each other (about 0.1 pF). For example, when moving images aredisplayed at 60 fps, the hold time for one signal writing can beapproximately 160 seconds, which is 10000 times that of the transistorincluding low-temperature polysilicon. In this manner, still images canbe displayed on a display portion even by less frequent writing of imagesignals.

Embodiment 4

In this embodiment, an example of a display device which is oneembodiment of the present invention is described.

FIG. 14A illustrates an example of a display device including the shiftregister circuit in Embodiment 2. The display device illustrated in FIG.14A includes a timing controller 5360, a driver circuit 5361 having, asource driver circuit 5362, a gate driver circuit 5363_1, and a gatedriver circuit 5363_2, and a pixel portion 5364. A plurality of sourcesignal lines 5371 which extend from the source driver circuit 5362 and aplurality of gate signal lines 5372 which extend from the gate drivercircuits 5363_1 and 5363_2 are provided in the pixel portion 5364.Pixels 5367 are provided in matrix in regions where the plurality ofsource signal lines 5371 and the plurality of gate signal lines 5372intersect with each other.

Note that the display device can include a lighting device, a controlcircuit thereof, and the like. In that case, the pixel 5367 preferablyincludes a liquid crystal element.

Note that it is possible not to provide one of the gate driver circuit5363_1 and the gate driver circuit 5363_2.

The timing controller 5360 has a function of controlling the operationof the driver circuit 5361 by supplying a control signal to the drivercircuit 5361. For example, the timing controller 5360 supplies a controlsignal such as a start signal SSP, a clock signal SCK, an inverted clocksignal SCKB, a video signal DATA, or a latch signal LAT to the sourcedriver circuit 5362. Further, the timing controller 5360 supplies acontrol signal such as a start signal GSP, a clock signal GCK, or aninverted clock signal GCKB to the gate driver circuit 5363_1 and thegate driver circuit 5363_2.

The source driver circuit 5362 has a function of outputting videosignals to the plurality of source signal lines 5371. The source drivercircuit 5362 can be referred to as a driver circuit, a signal linedriver circuit, or the like. Video signals are input to the pixels 5367.Display elements included in the pixels 5367 express gradation inaccordance with the video signals.

The gate driver circuit 5363_1 and the gate driver circuit 5363_2 eachhave a function of sequentially selecting the pixels 5367 in rows. Eachof the gate driver circuit 5363_1 and the gate driver circuit 5363_2 canbe referred to as a driver circuit or a scan line driver circuit. Timingof selecting the pixels 5367 is controlled when the gate driver circuit5363_1 and the gate driver circuit 5363_2 output gate signals to thegate signal lines 5372.

Note that in the display device illustrated in FIG. 14A, the gate drivercircuit 5363_1 and the gate driver circuit 5363_2 can be formed over thesame substrate as the pixel portion 5364. FIG. 14B illustrates anexample of the case where the gate driver circuit 5363_1 and the gatedriver circuit 5363_2 are formed over the same substrate as the pixelportion 5364 (a substrate 5380). Note that the substrate 5380 and anexternal circuit are connected to each other through a terminal 5381.

Note that in the display device illustrated in FIG. 14A, part of thesource driver circuit 5362 (e.g., a switch, a multiplexer, a shiftregister circuit, a decoder circuit, an inverter circuit, a buffercircuit, and/or a level shifter circuit) can be formed over the samesubstrate as the pixel portion 5364. FIG. 14C illustrates an example ofthe case where the gate driver circuit 5363_1, the gate driver circuit53632, part of the source driver circuit 5362 (denoted by a referencenumeral 5362 a) are formed over the same substrate as the pixel portion5364 (the substrate 5380) and another part of the source driver circuit5362 (denoted by a reference numeral 5362 b) is formed over a substratewhich is different from the substrate 5380.

The shift register circuit in Embodiment 2 can be used as the drivercircuit of the display device or part of the driver circuit. Inparticular, when the driver circuit of the display device includes thetransistor described in Embodiment 3, the usage of the shift registercircuit in Embodiment 2 enables improvement in the drive capability ofthe driver circuit. Thus, the display device can be made large.Alternatively, the resolution of the display portion can be improved.Alternatively, the layout area of the driver circuit can be reduced;thus, the frame of the display device can be reduced.

Embodiment 5

In this embodiment, examples of electronic devices are described.

FIGS. 15A to 15H and FIGS. 16A to 16D illustrate electronic devices.These electronic devices can include a housing 5000, a display portion5001, a speaker 5003, an LED lamp 5004, operation keys 5005 (including apower switch or an operation switch), a connection terminal 5006, asensor 5007 (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, smell, or infrared ray), a microphone 5008, and the like.

FIG. 15A illustrates a mobile computer, which can include a switch 5009,an infrared port 5010, and the like in addition to the above objects.FIG. 15B illustrates a portable image reproducing device provided with amemory medium (e.g., a DVD reproducing device), which can include asecond display portion 5002, a memory medium read portion 5011, and thelike in addition to the above objects. FIG. 15C illustrates agoggle-type display, which can include the second display portion 5002,a support 5012, an earphone 5013, and the like in addition to the aboveobjects. FIG. 15D illustrates a portable game machine, which can includethe memory medium read portion 5011 and the like in addition to theabove objects. FIG. 15E illustrates a projector, which can include alight source 5033, a projector lens 5034, and the like in addition tothe above objects. FIG. 15F illustrates a portable game machine, whichcan include the second display portion 5002, the memory medium readportion 5011, and the like in addition to the above objects. FIG. 15Gillustrates a television receiver, which can include a tuner, an imageprocessing portion, and the like in addition to the above objects. FIG.15H illustrates a portable television receiver, which can include acharger 5017 capable of transmitting and receiving signals and the likein addition to the above objects. FIG. 16A illustrates a display, whichcan include a support base 5018 and the like in addition to the aboveobjects. FIG. 16B illustrates a camera, which can include an externalconnection port 5019, a shutter button 5015, an image reception portion5016, and the like in addition to the above objects. FIG. 16Cillustrates a computer, which can include a pointing device 5020, theexternal connection port 5019, a reader/writer 5021, and the like inaddition to the above objects. FIG. 16D illustrates a mobile phone,which can include an antenna, a tuner of one-segment (1seg digital TVbroadcasts) partial reception service for mobile phones and mobileterminals, and the like in addition to the above objects.

The electronic devices illustrated in FIGS. 15A to 15H and FIGS. 16A to16D can have a variety of functions, for example, a function ofdisplaying a lot of information (e.g., a still image, a moving image,and a text image) on a display portion; a touch panel function; afunction of displaying a calendar, date, time, and the like; a functionof controlling processing with a lot of software (programs); a wirelesscommunication function; a function of being connected to a variety ofcomputer networks with a wireless communication function; a function oftransmitting and receiving a lot of data with a wireless communicationfunction; a function of reading a program or data stored in a memorymedium and displaying the program or data on a display portion. Further,the electronic device including a plurality of display portions can havea function of displaying image information mainly on one display portionwhile displaying text information on another display portion, a functionof displaying a three-dimensional image by displaying images whereparallax is considered on a plurality of display portions, or the like.Furthermore, the electronic device including an image receiving portioncan have a function of photographing a still image, a function ofphotographing a moving image, a function of automatically or manuallycorrecting a photographed image, a function of storing a photographedimage in a memory medium (an external memory medium or a memory mediumincorporated in the camera), a function of displaying a photographedimage on the display portion, or the like. Note that functions which canbe provided for the electronic devices illustrated in FIGS. 15A to 15Hand FIGS. 16A to 16D are not limited them, and the electronic devicescan have a variety of functions.

FIG. 16E illustrates an example in which a display device isincorporated in a building structure. FIG. 16E illustrates a housing5022, a display portion 5023, a remote controller 5024 which is anoperation portion, a speaker 5025, and the like. The display device isincorporated in the building structure as a wall-hanging type and can beprovided without requiring a large space.

FIG. 16F illustrates another example in which a display device isincorporated in a building structure. A display panel 5026 isincorporated in a prefabricated bath unit 5027, so that a bather canview the display panel 5026.

Note that although this embodiment describes the wall and theprefabricated bath unit as examples of the building structures, thisembodiment is not limited to them. The display devices can be providedin a variety of building structures.

Next, examples in which display devices are incorporated in movingobjects are described.

FIG. 16G illustrates an example in which a display device isincorporated in a car. A display panel 5028 is incorporated in a carbody 5029 of the car and can display information related to theoperation of the car or information input from inside or outside of thecar on demand. Note that the display panel 5028 may have a navigationfunction.

FIG. 16H illustrates an example in which a display device isincorporated in a passenger airplane. FIG. 16H illustrates a usagepattern when a display panel 5031 is provided for a ceiling 5030 above aseat of the passenger airplane. The display panel 5031 is incorporatedin the ceiling 5030 through a hinge portion 5032, and a passenger canview the display panel 5031 by stretching of the hinge portion 5032. Thedisplay panel 5031 has a function of displaying information by theoperation of the passenger.

Note that although bodies of a car and an airplane are illustrated asexamples of moving objects in this embodiment, this embodiment is notlimited to them. The semiconductor devices can be provided for a varietyof objects such as two-wheeled vehicles, four-wheeled vehicles(including cars, buses, and the like), trains (including monorails,railroads, and the like), and vessels.

The shift register circuit in Embodiment 2 is preferably incorporated inthe electronic device described in this embodiment. In particular, theshift register circuit in Embodiment 2 is preferably incorporated as acircuit for driving the display portion of the electronic device. Whenthe shift register in Embodiment 2 is incorporated as a circuit fordriving the display portion of the electronic device, the area of adriver circuit can be reduced and the size of the display portion can beincreased. Further, the resolution of the display portion can beimproved.

This application is based on Japanese Patent Application serial no.2010-033669 filed with Japan Patent Office on Feb. 18, 2010, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A circuit implementing a circuit diagramcomprising: a first transistor; a second transistor; a third transistor;and a fourth transistor, wherein, in the circuit diagram: a firstterminal of the first transistor is electrically connected to a firstwiring; a second terminal of the first transistor is electricallyconnected to a second wiring; a first terminal of the second transistoris electrically connected to a third wiring; a first terminal of thethird transistor is electrically connected to a fourth wiring; a firstterminal of the fourth transistor is electrically connected to the thirdwiring; a gate electrode of the first transistor is directly connectedto a second terminal of the second transistor and a gate electrode ofthe fourth transistor; a gate electrode of the second transistor isdirectly connected to a second terminal of the third transistor and asecond terminal of the fourth transistor; and a gate electrode of thethird transistor is directly connected to the first terminal of thethird transistor, wherein a channel forming region of each of the firstto fourth transistors includes an oxide semiconductor, wherein the oxidesemiconductor comprises indium, gallium and zinc, and wherein anoff-state current of each of the first to fourth transistors is 1 aA/μmor less.
 2. The circuit according to claim 1, wherein carrier density ofthe channel forming region of each of the first to fourth transistors isless than 1×10¹¹/cm³.
 3. A display device including the circuitaccording to claim
 1. 4. A semiconductor device comprising the circuitaccording to claim
 1. 5. A semiconductor device comprising: a firsttransistor; a second transistor; a third transistor; and a fourthtransistor, wherein: a first terminal of the first transistor iselectrically connected to a first wiring; a second terminal of the firsttransistor is electrically connected to a second wiring; a firstterminal of the second transistor is electrically connected to a thirdwiring; a first terminal of the third transistor is electricallyconnected to a fourth wiring; a first terminal of the fourth transistoris electrically connected to the third wiring; a gate electrode of thefirst transistor is directly connected to a second terminal of thesecond transistor and a gate electrode of the fourth transistor; a gateelectrode of the second transistor is directly connected to a secondterminal of the third transistor and a second terminal of the fourthtransistor; and a gate electrode of the third transistor is directlyconnected to the first terminal of the third transistor, wherein achannel forming region of each of the first to fourth transistorsincludes an oxide semiconductor, wherein the oxide semiconductorcomprises indium, gallium and zinc, and wherein an off-state current ofeach of the first to fourth transistors is 1 aA/μm or less.
 6. Thesemiconductor device according to claim 5, wherein carrier density ofthe channel forming region of each of the first to fourth transistors isless than 1×10¹¹/cm³.
 7. A display device including the semiconductordevice according to claim
 5. 8. A circuit implementing a circuit diagramcomprising: a first transistor; a second transistor; a third transistor;and a fourth transistor, wherein, in the circuit diagram: a firstterminal of the first transistor is electrically connected to a firstwiring; a second terminal of the first transistor is electricallyconnected to a second wiring; a first terminal of the second transistoris electrically connected to a third wiring; a first terminal of thethird transistor is electrically connected to a fourth wiring; a firstterminal of the fourth transistor is electrically connected to the thirdwiring; a gate electrode of the first transistor is directly connectedto a second terminal of the second transistor and a gate electrode ofthe fourth transistor; a gate electrode of the second transistor isdirectly connected to a second terminal of the third transistor and asecond terminal of the fourth transistor; a gate electrode of the thirdtransistor is directly connected to the first terminal of the thirdtransistor; and the first wiring has a width which is smaller than achannel width of the first transistor.
 9. The circuit according to claim8, wherein the width is larger than a width of at least a part of thefourth wiring.
 10. The circuit according to claim 8, wherein a channelforming region of each of the first to fourth transistors includes anoxide semiconductor.
 11. The circuit according to claim 8 whereinoff-state current of each of the first to fourth transistors is 1 aA/μmor less.
 12. The circuit according to claim 8, wherein carrier densityof a channel forming region of each of the first to fourth transistorsis less than 1×10¹¹/cm³.
 13. A semiconductor device comprising thecircuit according to claim
 8. 14. A display device including the circuitaccording to claim
 8. 15. A semiconductor device comprising: a firsttransistor; a second transistor; a third transistor; and a fourthtransistor, wherein: a first terminal of the first transistor iselectrically connected to a first wiring; a second terminal of the firsttransistor is electrically connected to a second wiring; a firstterminal of the second transistor is electrically connected to a thirdwiring; a first terminal of the third transistor is electricallyconnected to a fourth wiring; a first terminal of the fourth transistoris electrically connected to the third wiring; a gate electrode of thefirst transistor is directly connected to a second terminal of thesecond transistor and a gate electrode of the fourth transistor; a gateelectrode of the second transistor is directly connected to a secondterminal of the third transistor and a second terminal of the fourthtransistor; a gate electrode of the third transistor is directlyconnected to the first terminal of the third transistor; and the firstwiring has a width which is smaller than a channel width of the firsttransistor.
 16. The semiconductor device according to claim 15, whereinthe width is larger than a width of at least a part of the fourthwiring.
 17. The semiconductor device according to claim 15, wherein achannel forming region of each of the first to fourth transistorsincludes an oxide semiconductor.
 18. The semiconductor device accordingto claim 15, wherein off-state current of each of the first to fourthtransistors is 1 aA/μm or less.
 19. The semiconductor device accordingto claim 15, wherein carrier density of a channel forming region of eachof the first to fourth transistors is less than 1×10¹¹/cm³.
 20. Adisplay device including the semiconductor device according to claim 15.